I am currently doing an EU4 game as Brandenburg > Prussia > Germany on Ironman. It's really fun to date.
Ironman makes it so you can't save scum which means you have to deal with whatever happens. I quite like this. I had a successful debut with taking my Neumark core from the Teutons helped by quick alliances to Poland, Austria and Bohemia and marriages to both (I knew Poland would betray me after taking Danzig so I didn't marry them). After that, I did the Pomeranian succession mission, took Hinterpommern for myself and vassalized the rest of the country. I used Hinterpommern to fake a claim on Danzig (still Teutonic) then sold it back to Pomerania. I declared again on the Teutonic Order, my allies came forward again, so I took Danzig and vassalized the rest of the country. I gave Danzig to Pomerania. In this way, they would core it for me and when I annex them, I would regain the province (and Hinterpommern) without spending any admin points.
Poland betrays me and starts hating me as expected. They are allied to Denmark, who has integrated Norway and is still in a union with Sweden, and they have a union themselves with Lithuania. Polithuania is already really strong, and so is Swedenmark. The two together are scary indeed. They declare on me and the only ally that joins me in defence is Bohemia. I only have my two vassals and the Czech pornographers in my corner. With Austria, this could have been winnable, but we're now totally screwed. Thanks a lot Austria. They make short work of our armies and land about 50-60k troops (this is before 1500, so that's a lot) to carpet siege us. I make a quick peace, giving a Bohemian province to Poland and cutting my Teuton vassal. I also give them all my money and 10% on my future earnings for 10 years. God damn it.
To make matters worse, somehow my truce with the Teutons is longer than theirs, so I can't revassalize them right away. They invade the Teutons, Denmark taking most of the Baltic coastal provinces and Poland, the rest, while I look in horror. Ostpreussen, a province I need to form Prussia, has been taken by Denmark, who looks well on her way to integrate Sweden and form Scandinavia. I can't compete with them and my allies now rarely join my wars, declaring wars on their own. I almost restart, but I fight on.
I integrate Pomerania into my country. To expand, I decide to face the other way : West instead of East. German provinces are quite rich so I surmise that this will help my fight against the Northern bastards and the Kielbasa fuckers. I take a few, going as far as Stade. I do my utmost to maintain my alliance with Austria, so they don't hate me for annexing HRE members. The Hansa is very very powerful somehow and I can't take it, so I unfortunately have to skip Lübeck. I force a new one with France, along with a new one with Muscovy.
Checking back on Denmark again, I notice that Sweden finally declared its independence and is occupying large swathes of Danish territory in the Scandinavian mainland. I let that happen. Poland has lost its union with Lithuania also, but Lithuania is allied with Denmark, along with Muscovy. I'm stuck in another impossible web of alliances, and I fear I can't take advantage of Denmark's troubles...or can I?
Denmark may be allied with Lithuania and Muscovy, but Poland is only allied to Denmark and Muscovy. I declare war on Poland to wage war with Denmark by proxy instead of Denmark proper. As Denmark then cannot call its own allies in the war unless I name it as a co-belligerent, this frees up Lithuania for neutrality at least, or warring on my side at the most, but in no case can it be called to fight me. The caveat is that I have to pay double warscore to take Danish lands, and I want Ostpreussen. France and Austria join me along with Lithuania. Lithuania acts as a bulwark against Muscovy, without which the war would have been much harder. As they are allied with the party on the defensive, it is them that they join and not me. Denmark's army was already ravaged by Sweden, who will become independent during the war, and the Polish army cannot stand against our combined might while Lithuania holds the line against Muscovy. My allies then go fight the Vodka drunks while I siege the provinces Denmark has in the Baltic. I then siege some Polish border provinces. After being done with Muscovy, my allies siege most of the rest. I try to peace out Denmark, but due to the double cost, they don't even want to give me Ostpreussen. Bastards. I siege and occupy their capital to make a point, and they relent. I get my Ostpreussen, then I peace out Poland by taking 3 provinces for myself and giving back the Bohemian core they took last time to Bohemia. Never let it be said that I am not magnanimous. I core the Polish stuff and Ostpreussen. I now have every required province to form Prussia, but I don't have the right religion!
Also, my realm is unfortunately disjointed because the Danish hold Ermland, the single province between my territory and my Ostpreussen exclave. I don't like this state of things.
The Religious Troubles now start. One of my provinces is converted to Protestantism. I am currently in a war where I'm sitting on my ass just not to break an alliance and it's taking forever to finish ;_; however, it finally does, and I change my state religion to Protestant. I convert every one of my Catholic provinces in a short time. Bohemia hates me because I refused a Call to Arms when I had no manpower and barely an army...I wish the AI would stop calling you in when you're already weak. I declared on them because they had OPM allies only and seize a few provinces, but I got gypped on dice rolls and lost. Fuck.
Meanwhile, Demark has regained some Swedish lands, but Norway declared independence and Denmark is having a LOT of trouble with the Religious Turmoils...there are at least 3x30 stacks of Catholic zealots sieging its provinces. However...there is also 2x19 stacks of Teuton nationalists! I fund them to make them friendly then wait. Eventually, Denmark can't stop them and Ermland secedes as the reformed Teutonic Order. They love me a lot for funding them, so I move to ally them to diplovassalize...but I'm yet again in a war I don't care about just to maintain an alliance, so I cannot ally them. Very well, I wait.
Poland invades the Teutons for Ermland while I'm waiting for the other war to conclude. FUCKING KIELBASA DILDOS! I cannot stand by and let them steal my land AGAIN. The AI called nobody to arms because it's just a single province minor, so Poland thought it'd have it easy. It's just Poland versus the Teutons...so I try to enforce a white peace on the side of the Teutons because I knew that Poland would reject, and used this casus belli to jump in defence of the Teutons. Poland now calls in the UK after my joining, but no other ally. The UK attempts a 18 stack drop on Ermland, but I'm there before them and they crush themselves on my now substantial army, thanks to Offensive ideas. The landing malus on their dice rolls extinguishes their stack. Poland attempts to attack me in Ermland, but I beat them back. The UK tries a few more landings but I always intercept them, winning all the battles. The UK finally GTFOs. Poland and I are now out of manpower, but I am richer. I hire mercenaries and destroy Poland's army for good. I then siege provinces near Ermland. It takes a while before the Teutons, the war leaders, sue for peace. They take another Polish province for themselves.
After that, I vassalize the Teutons and this is how my game looks at the moment, including an Europe screenshot. Once I integrate the Teutons, my realm will finally be joined.
My plans are now to take on The Hansa and Bohemia, and perhaps to finish kicking Poland's ass. I will also get Denmark's holdings on the Baltic coast. Since I now have The Goose Step, I will fill the Offensive tree and then take Quality or Defensive to make Prussian OP armies. Since I am Protestant, I can't become the Emperor yet, but I'm hoping a league war will be declared so that I may. Before all of this, I will go down in HRE lands and take the provinces required to form Germany.

